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United States Patent |
5,035,742
|
Uemura
,   et al.
|
July 30, 1991
|
Reduced chromium-ore bearing powder and method for producing the same
Abstract
A reduced chromium-ore bearing powder used for production of a
chromium-containing steel in a converter, is produced by a reduction of
chromium ore-powder having a particle-diameter of 3 mm or less by a
carbonaceous reducing agent having a particle diameter of 3 mm or less in
an inert-gas atmosphere, while the chromium-ore powder and carbonaceous
reducing agent are stirred and mixed with each other in the reaction
chamber (5).
The reduced chromium-ore powder has 3 mm or less of particle diameter.
Acid-soluble chromium is in an amount of 85% or more of the total
chromium, and acid-soluble iron is in an amount of 95% or more of the
total iron.
Inventors:
|
Uemura; Tadashi (Yamaguchi, JP);
Minagawa; Tsutomu (Yamaguchi, JP);
Saito; Sadahiro (Yamaguchi, JP)
|
Assignee:
|
Showa Denko K.K. (Tokyo, JP);
Shunan Denko K.K. (Tokyo, JP)
|
Appl. No.:
|
444162 |
Filed:
|
January 13, 1989 |
PCT Filed:
|
March 9, 1989
|
PCT NO:
|
PCT/JP89/00256
|
371 Date:
|
November 13, 1989
|
102(e) Date:
|
November 13, 1989
|
PCT PUB.NO.:
|
WO89/08724 |
PCT PUB. Date:
|
September 21, 1989 |
Foreign Application Priority Data
Current U.S. Class: |
75/623 |
Intern'l Class: |
B01J 002/04 |
Field of Search: |
75/623
|
References Cited
U.S. Patent Documents
3872193 | Mar., 1975 | Smith | 75/334.
|
4150975 | Apr., 1979 | Miyake | 75/623.
|
Primary Examiner: Rosenberg; Peter D.
Attorney, Agent or Firm: Sughrue, Mion, Zinn Macpeak & Seas
Claims
We claim:
1. A reduced chromium-ore bearing powder used for production of a
chromium-containing steel in a converter, which powder essentially
consists of a reduced-chromium ore, and free carbon, wherein said reduced
chromium-ore essentially consists of an acid-soluble chromium, a chromium
oxide, an acid-soluble iron, an iron oxide, and gangue material, and,
further said reduced chromium-ore powder has 3 mm or less of particle
diameter, said acid-soluble chromium is in an amount of 85% or more of the
total chromium, and said acid-soluble iron is in an amount of 95% or more
of the total iron.
2. A reduced chromium-ore bearing powder according to claim 1, wherein said
free carbon is in an amount of from 3 to 10% by weight based on said
powder.
3. A reduced chromium-ore bearing powder according to claim 2, wherein the
total chromium is in an amount of from 22 to 48% by weight and the total
iron is in an amount of from 11 to 24% by weight of said powder.
4. A reduced chromium-ore bearing powder according to any one of claims 1
through 3, wherein it is produced by a reduction of chromium ore-powder
having a particle-diameter of 3 mm or less by a carbonaceous reducing
agent having a particle diameter of 3 mm or less in an inert-gas
atmosphere.
5. A reduced chromium-ore bearing powder according to claim 4, wherein said
chromium-ore powder and said carbonaceous reducing agent are stirred and
mixed with each other during the reduction.
6. A method for producing a reduced chromium-ore bearing material by means
of reducing chromium ore with a carbonaceous reducing agent, comprising:
stirring and mixing said chromium ore having a particle diameter of 3 mm
or less and said carbonaceous reducing agent having a particle diameter of
3 mm or less, in an amount at least equal to the amount needed for
reducing the chromium oxide and iron oxide contained in the chromium ore;
and, heating said chromium ore and carbonaceous reducing agent to a
temperature of from 1200.degree. to 1500.degree. C. in an inert-gas
atmosphere, while said chromium ore and carbonaceous reducing agent are
stirred and mixed.
7. A method for producing a reduced chromium-ore bearing material according
to claim 6, wherein said stirring and mixing is carried out in a rotary
furnace which comprises the following rotary members capable of rotating
therewith and being integral therewith: a reaction chamber (5) located at
the center of the rotary furnace (20) and defined by polygons in cross
section made of heat resistant ceramics (4); and, a plurality of
heating-gas chambers (6) formed around the reaction chamber (5).
8. A method according to claim 6 or 7, wherein said inert-gas atmosphere is
a CO gas atmosphere which is formed as a result of the reaction between
said chromium ore and said carbonaceous reducing agent.
Description
DESCRIPTION
1. Technical Field
The present invention relates to a reduced chromium-ore bearing powder and
a method for producing the same. More particularly, the present invention
relates to a highly reduced chromium-ore bearing powder which is used for
producing a chromium-containing steel, such as stainless steel, in a
converter, and which is suitable for conveyance by carrier gas and is
directly blown into the molten steel in the steel making process.
2. Background Art
Various methods have been devised for producing at low cost the
chromium-bearing raw material of stainless steel. Merits and demerits of
such methods are greatly influenced by the conditions of raw materials and
electric power and by condition of location of a smelting plant. It is
crucial, in Japan for example, to effectively utilize powdered chromium
ore which has a poor grade, in order to minimalize production costs.
Incidentally, developments are being made on how to produce a stainless
steel by means of blowing chromium-ore powder into an oxygen top-and/or
bottom-blowing converter for steel making. Fundamental reaction in a
converter oxidizes and removes carbon contained in molten pig iron with
the aid of oxygen. Combustion heat is obtained by the oxidation and is
utilized to elevate the temperature of molten steel. Upon injection of the
chromium-ore powder into the molten steel, the chromium ore must not only
be melted but also be reduced. Chromium ore must be first melted, and then
the reduction of chromium ore occurs in the molten state. Heat source is
indispensable for melting and reduction. A carbonaceous agent is usually
added into a converter, and is utilized as both a reducing agent and heat
source. In order that combustion of the carbonaceous agent take place,
oxygen is necessary, with the result that the amount of oxygen blown
increases, and the refining time becomes considerably longer. In a more
metallurgical aspect, the addition of a carbonaceous agent into a
converter necessitates simultaneous oxidation (combustion) of carbon and
reduction of ore. There is a limitation as to whether both the oxidation
and reduction reactions can proceed in an identical converter. In order to
thoroughly reduce the chromium ore in a converter, the amount of reducing
agent is considerably excessive more than the chemical equivalent amount
for reducing the chromium ore added in a converter, with the result that
productivity is decreased and cost is increased. In most of the steel
making plants, a continuous-continuous casting is carried out. In this
case, refining time matches casting time. When a carbonaceous reducing
agent is added into a converter, the continuous-continuous casting is
carried out with difficulty, with the result that such disadvantages as
decrease in productivity and recovery and increase in labor are incurred.
Blowing of reduced chromium-ore bearing powder appears to overcome the
difficulties involved in the addition of chromium ore. The following
methods for producing the reduced chromium-ore bearing powder are known.
(1) Chromium ore, carbonaceous reducing agent and binder are agglomerated
into pellets having appropriate size and strength and are reduced by
heating in inert atmosphere (Japanese Examined Patent Publication No.
38-1959).
(2) Raw materials in the form of powder are stirred in a furnace which is
equipped with inner burners for the combustion of hydrocarbonaceous fuel
(U.S. Pat. No. 2,582,469).
(3) Raw materials in the form of powder are reduced by means of introducing
hydrocarbonaceous gas therethrough (Japanese Unexamined Patent Publication
No. 59-179725).
In method (1), in which pellets are produced and then reduced, the raw
materials in the form of powder must intentionally be once pelletized and
subsequently be again crushed to obtain powder. The production of pellets
and crushing is complicated and results in increase in cost. In addition,
in order to fulfill the certain strength requirement of the pellets,
limitations are imposed upon the raw materials and production methods of
pellets, and hence result in increase in cost.
In method (2), in which by use of inner burners, combustion of the
hydrocarbonaceous fuel takes place, the inner atmosphere of a furnace
contains an oxidizing stream, such as CO.sub.2 formed due to combustion by
the burners. In the case of pellets, only their surfacial parts are
re-oxidized and hence a certain degree of reduction, for example 80%, is
obtained. In the case of powder, since it has a large specific surface
area, the extent of re-oxidation becomes higher, and hence the reduction
degree remains low, for example 60% at the highest.
In method (3), in which the reducing gas and chromium in the form of powder
are brought into contact, the reduction occurs in a gas phase-solid phase
reaction. In order to thoroughly bring the gas and powder into contact
with one another, the ore must be fluidized satisfactorily, with the
result that the construction of a plant becomes complicated, and further,
the temperature cannot be elevated to a high level. The reduction degree
is accordingly suppressed at a low level. In addition, since the
hydrocarbon is expensive, the cost is increased.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide a reduced chromium-ore
bearing powder which has a high content of reduced chromium and incidental
iron, and is hence suitable for adding into a converter, in which the
oxidation with pure oxygen is the predominant reaction.
It is another object of the present invention to provide a method for
producing a reduced chromium-ore bearing powder, wherein a high reduction
degree is attained without incurring increase in cost, as compared with
the known methods.
In accordance with the objects of the present invention, there is provided
a reduced chromium-ore bearing powder used for production of a
chromium-containing steel in a converter, which powder essentially
consists of a reduced-chromium ore and free carbon, wherein said reduced
chromium-ore essentially consists of an acid-soluble chromium, a chromium
oxide, an acid-soluble iron, an iron oxide, and gangue material, and,
further the reduced chromium-ore powder has 3 mm or less of particle
diameter, the acid-soluble chromium is in an amount of 85% or more of the
total chromium, and the acid-soluble iron is in an amount of 95% or more
of the total iron.
In accordance with the present invention, there is also provided a method
for producing a reduced chromium-ore bearing material by means of reducing
chromium ore with a carbonaceous reducing agent, characterized by:
stirring and mixing the chromium ore having a particle diameter of 3 mm or
less and the carbonaceous reducing agent having a particle diameter of 3
mm or less, in an amount of at least equal to the equivalent amount for
reducing the chromium oxide and iron oxide contained in the chromium ore;
and, heating the chromium ore and carbonaceous reducing agent to a
temperature of from 1200.degree. to 1500.degree. C. in an inert
gas-atmosphere, while said chromium ore and carbonaceous reducing agent
are stirred and mixed.
The stirring and mixing is preferably carried out in a rotary furnace which
comprises the following rotary members capable of rotating therewith and
being integral therewith: a reaction chamber located at the center of the
rotary furnace and defined by polygons in cross section made of heat
resistant ceramics; and, a plurality of heating-gas chambers formed around
the reaction chamber.
BRIEF DESCRIPTIONS OF DRAWINGS
FIG. 1 is a lateral cross-sectional view of an example of an external
heating, rotary furnace used for carrying out the present invention.
FIG. 2 is a longitudinal cross-sectional view of FIG. 1.
FIG. 3 shows an experimental furnace.
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors carried out experiments by using the heating device
as shown in FIG. 3. A gas-tight reaction chamber 31 is rotatably mounted
in a furnace 32. Into a tubular crucible 34 made of graphite was charged
two kinds of raw materials 33. One kind was a mixture of chromium ore and
powder cokes, both having particle diameter of 3 mm or less. The
compositions of chromium ore and powder cokes are given in Table 1, below.
The other kind was prepared by crushing the chromium ore and powder cokes
having the same compositions as the one mentioned above to 90% passing
through 150 mesh, adding binder to the powder, and agglomerating the
powder to pellets 2.4 cm in diameter. Nitrogen gas was passed through the
core chamber 31 to create the inert atmosphere. Heating was carried out to
attain an inner temperature of 1300.degree. C. or more. For each of the
raw materials, the reaction chamber 31 was rotated and kept stationary, so
as to investigate the influence of rotation on the speed of reduction
reaction. The reduction degree of the chromium (%) are shown in Table 2.
TABLE 1
______________________________________
Fixed Volatile
Cr.sub.2 O.sub.3
FeO Carbon Matters
Ash Gangue
______________________________________
Chromium
45.7 25.4 -- -- -- 28.0
ore
Coal -- -- 57.3 34.7 8.0 --
Cokes -- -- 87.5 1.5 11.0 --
______________________________________
TABLE 2
______________________________________
Reaction
Time (hrs)
20 40 60 80
______________________________________
Powder Stationary 6.0 13.5 18.7 25.3
Raw Stir 47.1 60.5 68.8 74.7
Materials
Pellets Stationary 60.5 74.7 82.4 87.3
Stir 61.8 73.9 80.3 --
______________________________________
As is shown in Table 2, the reaction speed is high both in the stirring
case and the stationary case when using the pellets, while when raw
materials in the form of powder are used, the reaction speed is very slow
in the stationary case but is as high as the pellets in the stirring case.
The present invention is based on this discovery.
Various gases may be employed for creating the inert atmosphere in the
furnace. However, it is not necessary to blow particular gas into the
furnace. When the reaction is carried out in a closed furnace, the CO gas
formed as a result of the reaction can create the inert atmosphere.
Means for heating the furnace may be any appropriate one which does not
cause oxidation in the furnace-interior, such as installing electric
heaters within a closed furnace, or indirectly heating the furnace by
means of external burners. In the latter method of indirect heating, since
the temperature required for reducing the chromium ore is rather high, it
is considerably difficult to construct a furnace which exhibits enough
strength to reach a sufficiently high temperature for stirring the
chromium ore. For the indirect heating, a rotary furnace which comprises
the following rotary members capable of rotating therewith and being
integral therewith is recommended: a reaction chamber located at the
center of the rotary furnace and defined by polygons in cross section made
of heat resistant ceramics; and, a plurality of heating-gas chambers
formed around the reaction chamber.
A reduced chromium-ore bearing powder according to an embodiment of the
present invention contains free carbon in an amount of from 3 to 10% by
weight based on said powder.
A reduced chromium-ore bearing powder according to another embodiment of
the present invention contains the total chromium in an amount of from 22
to 48% by weight and the total iron in an amount of from 11 to 24% by
weight of said powder.
The particle diameters of the raw materials of chromium ore and the reduced
chromium ore as well as the carbonaceous reducing agent are 3 mm or less,
because the reduced chromium-ore bearing powder, according to the present
invention, is produced by a reduction of chromium ore-powder while it is
in contact with the carbonaceous reducing agent during the stirring and
mixing in the furnace, and hence the contact area between them must be
kept high. The temperature is limited to a range of from 1200.degree. to
1500.degree. C., since at a temperature below 1200.degree. C. reduction of
chromium oxide does not progress sufficiently, and, further, at a
temperature above 1500.degree. C. the chromium ore softens and sticks to
the inner wall of a reaction chamber, thereby making operation difficult.
When the reduced chromium-ore bearing powder is blown into the molten steel
of a converter, since major parts of chromium and iron have been converted
to an acid-soluble state, that is chromium-iron carbide, chromium and iron
are melted in the molten pig iron or steel and form a homogeneous alloy
without undergoing any reduction. An excessive quantity of heat for
reduction reaction is therefore unnecessary. It is also possible to
decrease the carbon additive and the oxygen in the converter, because the
reduction degree in the reduced chromium-ore bearing powder is high. In
this regard, the free carbon remaining unoxidized in the reduced
chromium-ore bearing powder plays the role of the carbon additive and thus
allows the decrease of the carbon additive. Furthermore, the extension of
refining time in the converter due to the addition of chromium-bearing
material can be minimalized.
According to the method of the present invention, the chromium ore in the
form of powder and carbonaceous reducing agent in the form of powder are
mixed and stirred with each other under inert atmosphere at an appropriate
temperature. That is, the reduction reaction proceeds under inert
atmosphere while the chromium-ore powder and carbonaceous powder are mixed
and stirred with each other. High reduction degree is attained in the
powder state of chromium ore such that 85% or more of the total chromium
is converted to chromium carbide, that is acid-soluble chromium. Reduction
of iron proceeds preferentially as compared with the chromium reduction
and 95% or more of the total iron is converted to iron carbide, that is,
acid-soluble iron. Since the raw materials in a powder form are used in
the present invention, neither a pre-agglomerating process nor a
post-crushing process are required at all. The chromium source provided by
the present invention has a high degree of reduction and is inexpensive.
The present invention is further described with reference to FIGS. 1 and 2
illustrating an external heating, rotary furnace.
Referring to FIG. 1, an embodiment of the external heating type rotary
furnace according to the present invention is shown at a vertical cross
section with respect to a rotary axis. Referring to FIG. 2, the identical
furnace is shown at a cross section parallel to the rotary axis.
Heat-insulative bricks 2 are radially lined around the inner surface of the
cylindrical steel mantle 1.
Height of the heat-insulative bricks 2 is not uniform around the steel
mantle 1, but, the supporting bricks 3 are located at an appropriate
distance therebetween, e.g., every seventh brick in the embodiment shown
in FIG. 1. The supporting bricks 3 support the ceramic plates 4 which are
partition walls of the heating-gas chambers 6. A reaction chamber 5 having
polygonal form in cross section is therefore surrounded and defined by the
ceramic plates 4 and supporting bricks 3. In addition, a plurality of
heating-gas chambers 6 are formed around the reaction chamber 5 by the
heat-insulative bricks 2, supporting bricks 3, and ceramic plates 4.
The rotary furnace body 20 is supported by rollers 8 via rings 7 and is
driven by a power source (not shown) to make it rotate. The combustion
furnace 22 and panels 21 are connected with the rotary furnace body 20 to
form an integral structure. Namely, the rotary furnace body 20, combustion
furnace 22, and panels 21 as a whole constitute an integrally rotary
furnace body.
The rotary furnace body 20 is supported aslant in such a manner that the
end beside the panels 21 is elevated and forms a slight angle to the
horizontal plane. Pipes for feeding fuel and air are connected to the
burners 11 via universal joints not shown. The burners 11 are rotated
together with the rotary furnace body 20.
Since the reaction chamber 5 and heating-gas chambers 6 are constructed as
above, when the steel mantle 1 is rotated, they (5 and 6) are rotated
integrally with the rotation of steel mantle 1.
High temperature gas obtained in the combustion chamber 10 is passed
through the heating-gas chambers 6 of the rotary furnace body 20, which is
opposite the combustion chamber 10. The high temperature gas heats the
ceramic plates 4 of the partition walls while passing through the heating
gas chamber 6, and, after passing through exhaust gas prt 14, is collected
in exhaust gas-chamber 9, and is evetually let out of the outside heating
system through an exhaust gas-outlet 13. Meanwhile, materials to be
treated are fed through the raw materials supplying port 15 to the
reaction chamber 5 and are then subjected to rotary traveling in the
reaction chamber 5, while being indirectly heated by combustion gas which
is isolated from the materials. These materials now the (finished)
product, are then withdrawn, from the reaction chamber 5 through the
product-outlet 16 provided on the lower part of the combustion furnace 22.
The product is then collected via chute 17 and withdrawn.
For the heat-insulative brick, bricks having low heat conductivity are used
so as to attain the smallest external dissipation of heat through the
steel mantle. For practical purposes, conductivity (.lambda.) of
heat-insulative bricks is from 0.10-2.0 kcal/m.h. .degree. C.
(1000.degree. C.), preferably 0.1-0.5 kcal/m.h. .degree. C.
Heat-insulative bricks may be porous, e.g., have porosity ranging from 60
to 70%. The heat-insulative bricks may be constructed in dual layers.
Since the supporting bricks 3 are used for supporting the ceramic polygon,
high strength bricks should be used, even if it entails a sacrifice of
slight heat conductivity. Preferred bricks for the supporting bricks are
those based on schamotte and alumina. Brickwork of the heat-insulative
bricks 2 may be performed with the use of castable refractory.
The ceramics which form the polygon should have strength able to withstand
a high temperature of 1400.degree. C. or more and a high heat
conductivity, and should not be affected by combustion gas at a high
temperature. Materials satisfying these requirements are ceramics, such as
silicon carbide, aluminum nitride, alumina, and the like. Silicon carbide
is particularly preferred, since large sized sintering products are
available. Sintered silicon carbide exhibits a heat conductivity of 10
kcal/m.h. .degree. C. or more (at 1000.degree. C.), compression strength
(bending strength) of 200 kg/cm.sup.2 (at 1300.degree. C.) or more, and is
characterized as having high strength and high heat-conductivity. such
strength is satisfactory for supporting the load of the charged materials,
when exposed to combustion gas stream.
In an example described hereafter a furnace constructed as described above
was used. The specifications of the furnace were: inner diameter of iron
mantle--1300 mm; length of iron mantle--11 m; rotation number--0.12 rpm;
fuel of burners-- heavy oil; the highest temperature of the reaction wall
--1475.degree. C.: and, the length of a region of the reaction wall having
a temperature of 1200.degree. C. or more--7 m.
The powdered, chromium ore, cokes and coal having the compositions as shown
in Table 1 were weighed and blended in such a manner that the amount of
carbon is the same as that required for reducing 100% of the chromium ore.
The raw materials were charged through the inlet port into the reaction
chamber 5. The raw materials were rotated and stirred together with the
rotation of rotary furnace body 20. The raw materials were mixed and
successively displaced through the reaction chamber toward the outlet port
16 for withdrawing the product. During the displacement, the raw materials
were heated by direct contact with the partition wall made of ceramic
plates 4 and by radiation heat. The chromium ore in the form of powder and
carbonaceous reducing agent were forced to come in contact with one
another by the stirring. The points of contact were renewed due to the
stirring. The reduction reaction proceeded between the solid phases at the
contact points where the temperature rose to 1000.degree. C. or more.
The staying time of raw materials in the above described external heating,
rotary furnace was 6.8 hours. A total of 1.4 tons of sum of the raw
materials were treated per hour. The raw materials were heated to a
temperature of 1200.degree. C. or more for 1.9 hours in staying time. The
chemical analysis of the resultant products is shown in Table 3. The
reduction degrees of iron and chromium were 99% and 88.2%, respectively.
In comparison, the same reduction treatment as above was carried out with
the pellets. The pellets were prepared by finely crushing the raw
materials weighed and blended as described above to a size where 90% or
more pass through 200 mesh. Bentonite and water were added to the powder,
which was then pelletized to a diameter of 5 to 20 mm, followed by drying.
The reduction degree of iron and chromium were 97.8% and 93.6%,
respectively, as shown in Table 3.
TABLE 3
______________________________________
T.Cr Sol.Cr T.Fe Sol.Fe
T.C RR
______________________________________
Inventive
34.0 30.0 22.5 22.3 6.9 0.924
Comparative
34.4 32.2 22.6 22.1 4.8 0.949
______________________________________
RR = (A/B) .times. 100(%)
A = (Sol.Cr)/34.67 + (Sol.Fe)/55.85
B = (Total.Cr)/34.67 + (Total.Fe)/55.85
INDUSTRIAL APPLICABILITY
The reduced chromium-ore bearing powder according to the present invention
can be used for producing stainless steel and other chromium-containing
steel in a converter other metallurgical vessel where the predominant
reaction is oxidation. When a reduced chromium-ore bearing material having
a high reduction degree according to the present invention is charged in a
converter, a reduction reaction can be avoided.
In the method of the present invention, pelletizing is unnecessary. Heat
sources used in the present invention may be heavy oil or other fuels as
well as electric power. Therefore, the method according to the present
invention is appropriate for producing at a low cost a reduced
chromium-ore bearing powder having a high degree of reduction.
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